1. Technical Field of the Invention
The present invention relates generally to an automatic balancer for a rotating machine, and more particularly to an improved structure of a ball balancer designed to automatically equalize loads acting on a moving part of a rotating machine in a radius direction during rotation.
2. Background of Related Art
Conventional automatic balancers used in a rotating machine are of two different types: a liquid balancer having an annular casing filled with the liquid and a ball balancer having balls disposed within an annular casing.
FIGS. 21 and 22 show a conventional ball balancer, as taught in Japanese Patent Second Publication No. 56-130249, which includes a large number of balls 21 disposed within an annular casing 40 over 30% to 60% of the periphery of the annular casing 40. When the annular casing 40 spins at high speed, it will cause the balls 21 to be biased toward the opposite side of an unbalanced mass 15 to provide a counterbalance with the unbalance mass 15, thereby minimizing oscillation of the casing 40 during rotation.
A centrifuge, as an example of rotating machines, is designed to revolve a rotor at high speed to separate a mixture such as a liquid solution put in the rotor into a higher density component and a lower density component so that the higher density component may be settled away from the center of the rotor, while the lower density component may be settled around the center of the rotor. A sudden change in speed of the rotor during acceleration or deceleration will cause the separated components within the rotor to be stirred so that they are mixed. In order to avoid this problem, the acceleration or deceleration is controlled so as to be changed slowly during rotation of the rotor. In general, the centrifuge rotates at a high speed greater than a resonant speed initiating oscillation of the rotor. When the resonant speed is reached during a slow change in acceleration or deceleration of the rotor, it produces a great vibration of resonance frequency. To suppress this vibration, a damper is commonly installed between a drive mechanism for the rotor and the casing. However, when the centrifuge is operated to revolve the rotor which is unbalanced, it is difficult to absorb the vibration only using the damper, thus resulting in a large-scale vibration or noise caused by resonance of the rotor. Further, when a dynamic unbalance of the rotor continues during high-speed rotation, it will lead to eccentric rotation of the rotor, thereby increasing loads which bends a rotor shaft and damages a bearing of the rotor shaft. In order to avoid this problem, in a conventional centrifuge, a difference in weight between mixtures disposed on opposite sides of a rotor is minimized to counterbalance the rotor during rotation. This balance adjustment is usually made by adjusting the amounts of the mixtures or adding a balance weight to the rotor and wastes the time of an operator undesirably.
It is known in the art that a rotating system including a rotor generates great vibrations when the speed of the rotor reaches a resonant speed (i.e., when the rotor speed coincides with a natural frequency of the rotating system). When the rotor speed is less than the resonant speed, the center of gravity of the rotating system is offset from the center of the rotor. When the rotor speed exceeds the resonant speed, the phase of vibration of the rotating system is shifted 180.degree., so that the center of gravity of the rotating system is shifted to the center of rotation from the center of the rotor.
Therefore, when a centrifuge equipped with the ball balancer, as shown In FIGS. 21 and 22, is operated to accelerate a rotor, the rotor spins slowly while swinging toward an unbalanced mass until the rotor speed reaches the resonant speed, so that the balls are moved toward the unbalanced mass, resulting in an increase in dynamic unbalance of the rotor. Specifically, the resonance vibration becomes greater than would be the case without use of the ball balancer, which may cause the rotor to be brought into contact with an outer casing, resulting in unwanted mechanical noise.
When the rotor speed exceeds the resonant speed, the rotor swings in a direction opposite the unbalanced mass, thereby causing the balls to be collected on the opposite side of the unbalanced mass to decrease the unbalanced mass. This results in quick reduction in vibration of the rotor to obtain a dynamic balance of the rotor.
An increase in overall weight of the balls or diameter of the annular casing into order to increase an allowable unbalanced mass causes the resonance vibration of the rotor to be increased by the ball balancer even when an unbalanced mass is small. Specifically, it is difficult to reduce the vibration of the rotor by increasing the overall weight of the balls or diameter of the annular casing.
Accordingly, the conventional ball balancer, as shown in FIGS. 21 and 22, is effective to achieve a dynamic balance of the rotor when the rotor speed is above the resonant speed, but has the disadvantage that the ball balancer works to increase the vibration of the rotor when the rotor speed is less than the resonant speed.